Temperature compensated oscillators (TCXOs) used in various fields are in heavy use in portable mobile communication devices such as a cellular phone and so on in recent years. Generally, as this kind of temperature compensated oscillator, a crystal oscillator is widely used in which an oscillation circuit is constituted of a 10 MHz band AT cut quartz crystal (resonator) as an oscillation source and provided with a temperature compensation circuit using some kind of frequency variable means so that the temperature characteristic in a cubic curve of the AT cut quartz crystal is cancelled to stabilize the oscillating frequency.
Further, based on the configuration of the temperature compensation circuit, the temperature compensated oscillators are roughly divided into an analog temperature compensated oscillator and a digital temperature compensated oscillator.
For these kinds of temperature compensated oscillators, a reduction in size and weight and a reduction in price as well as stability of an oscillation output signal are desired.
A package configuration example of a micro surface-mounted temperature compensated oscillator is shown in FIG. 8.
This temperature compensated oscillator has a package (container) 10 which is constituted of a package main body 11, a welded ring 12, and a cover 13, to the inside of which a quartz crystal (resonator) 15, a MOS IC (integrated circuit) chip 16 constituting an oscillation circuit and a temperature compensation circuit which will be later described, and a circuit element 17 such as a chip capacitor or the like are attached and sealed.
The temperature compensated oscillator has a circuit configuration as shown in FIG. 9. An oscillation circuit 20 forms an inverter oscillation circuit in which the quartz crystal 15, an inverter 21, and a feedback resistor 22 are connected in parallel, and their both connection points are grounded via DC cut capacitors Cc and Cd and voltage-controlled variable capacitors 23 and 24 which are oscillation capacitors, respectively.
Further, an output line 25 which outputs a signal based on an oscillation output is led out of the connection point on the output side of the inverter 21 and connected to an output terminal 26. It should be noted that another piezoelectric element can also be used as the resonator in place of the quartz crystal 15.
Furthermore, a temperature detection circuit 18 which detects the temperature state near the quartz crystal 15 in the oscillation circuit 20 by a thermister or the like and a temperature compensation circuit 30 for keeping the frequency of the signal to be outputted to the output line 25 of the oscillation circuit 20 constant based on the temperature detection signal from the temperature detection circuit 18, are provided.
The temperature compensation circuit 30 comprises a compensation data storage circuit (non-volatile memory) 31 which stores compensation data for performing temperature compensation and a D/A conversion circuit 32 which generates a control voltage based on the compensation data and the temperature detection signal from the temperature detection circuit 18.
Then, the control voltage outputted from the D/A conversion circuit 32 is applied to the positive electrode side of the voltage-controlled variable capacitors 23 and 24 (the connection points with the DC cut capacitors Cc and Cd) via resistors R1 and R2 provided in the oscillation circuit 20 respectively, so as to change the capacitances of the voltage-controlled variable capacitors 23 and 24 in accordance with the voltage. This controls the oscillation frequency of the oscillation circuit 20 to keep the frequency of the output signal substantially constant.
In such temperature compensated oscillators, all of the quartz crystals 15 and the oscillation circuits 20 formed in the IC chips 16 cannot be formed completely the same due to variation in manufacturing or the like, and therefore they will individually have different temperature-frequency characteristics. Accordingly, all of the oscillation circuits 20 cannot be temperature-compensated based on the same reference.
Therefore, it is necessary to create individual compensation data different for each oscillation circuit and store it into the compensation data storage circuit 31. However, the oscillation circuits cannot be sufficiently compensated if the quartz crystals 15 exhibit a wide range of characteristic variation, and it is therefore necessary to adjust as much as possible the characteristics of the quartz crystals 15 in advance.
Hence, the adjustment work has conventionally been performed as in the following steps.
Step 1: Only the piezoelectric element such as the quartz crystal 15 or the like is mounted in the package (the package main body 11 in FIG. 8).
Step 2: The package is kept at a reference temperature (generally at room temperature: 25° C.), and an electrode film on the surface of the piezoelectric element is removed by an ion beam or the like while the resonant frequency of the piezoelectric element is being monitored by a network analyzer or the like to adjust the frequency to a desired frequency.
Step 3: The IC chip constituting the oscillation circuit and the temperature compensation circuit is mounted in the package.
Step 4: The package is exposed to a plurality of temperature states, and the oscillation frequency is measured in each of the temperature states to measure the difference with respect to the desired oscillation frequency f0.
Step 5: Based on the measurement values, temperature compensation data is created and stored into the compensation data storage circuit (non-volatile memory) of the IC chip.
As described above, in the conventional adjustment method of the temperature compensated oscillator, the IC chip constituting the oscillation circuit is not mounted when the characteristics of the piezoelectric element such as the quartz crystal or the like is adjusted, but the piezoelectric element is caused to resonate and its resonant frequency is monitored from the outside by the network analyzer or the like and the electrode film on the surface of the piezoelectric element is removed so that the frequency has a desired value.
Accordingly, there has been a problem that deviation arises between the oscillation frequency when the IC chip is also mounted in the package to make up the oscillation circuit together with the piezoelectric element and the circuit is caused to perform oscillation operation and the previously adjusted resonant frequency. In addition, the number of adjustment steps has been large which requires extra adjustment cost.
Hence, it is conceivable that the piezoelectric element and the IC chip are mounted in the package and the oscillation circuit is caused to operate, and its oscillation frequency is monitored so that adjustment of the resonant frequency of the piezoelectric element at room temperature and creation of the compensation data thereafter can be performed in sequence in a state close to the actual use state, in which case, however, the temperature compensation circuit also operates. In addition, no compensation data is stored in the temperature compensation data storage circuit in the initial state, but the initial value of the register for storing the data is unknown such that bits thereof are all “0” in some case and are all “1” in the other case. This brings about a problem that it is impossible to perform appropriate adjustment of the resonant frequency of the piezoelectric element such as the quartz crystal or the like and proper creation of the compensation data thereafter.